This application is related to application Ser. No. 229,366, an improvement on the instant invention, filed of even date by B.S. Saha et al., titled "Interdispersed Two-Phase Composite."
This invention relates to electrostatography and more particularly it relates to rare earth containing magnetic carrier particles and developers for the dry development of electrostatic charge images.
In electrostatography, an electrostatic charge image is formed on a dielectric surface, typically the surface of the photoconductive recording element. Development of this image is commonly achieved by contacting it with a two-component developer comprising a mixture of pigmented resinous particles, known as toner, and magnetically attractable particles, known as carrier. The carrier particles serve as sites against which the non-magnetic toner particles can impinge and thereby acquire a triboelectric charge opposite to that of the electrostatic image. During contact between the electrostatic image and the developer mixture, the toner particles are stripped from the carrier particles to which they had formerly adhered (via triboelectric forces) by the relatively strong electrostatic forces associated with the charge image. In this manner, the toner particles are deposited on the electrostatic image to render it visible.
It is known in the art to apply developer compositions of the above type to electrostatic images by means of a magnetic applicator which comprises a cylindrical sleeve of non-magnetic material having a magnetic core positioned within. The core usually comprises a plurality of parallel magnetic strips which are arranged around the core surface to present alternative north and south magnetic fields. These fields project radially, through the sleeve, and serve to attract the developer composition to the sleeve outer surface to form a brushed nap. Either or both the cylindrical sleeve and the magnetic core are rotated with respect to each other to cause the developer to advance from a supply sump to a position in which it contacts the electrostatic image to be developed. After development the toner depleted carrier particles are returned to the sump for toner replenishment.
As described in U.S. Patent Application Ser. No. 62,023, filed June 15, 1987 by B. S. Saha et al., titled "Electrographic Magnetic Carrier Particles," now U.S. Pat. No. 4,764,445, it was discovered that a hard magnetic ferrite material having a single phase hexagonal crystal structure could be formed which contained about 1 to about 5% by weight lanthanum. The lanthanum increased the conductivity of the material without adversely affecting its magnetic properties, resulting in superior magnetic carrier particles. The deleterious effect on magnetic properties was avoided only when a single phase crystal structure was formed, and magnetic properties were worsened when the lanthanum exceeded 5% and a single phase crystal structure was not formed. It is generally known that the conductivity of the carrier particles is directly proportional to the speed of development (the velocity of the photoconductive recording element over the magnetic brush) that can be employed, and a higher development speed means that more copies can be produced per unit time.
Attempts to form a single phase crystal structure using cerium (atomic number 57) instead of lanthanum (atomic number 56), did not result in a single phase crystal structure. Because cerium would not form a single phase crystal structure, other rare earths, farther from lanthanum in the Periodic Table, were also not expected to form single phase crystal structures.